1. Field of the Invention
The present invention relates to a coating apparatus and, more particularly, to an improvement in extrusion type coating apparatus that is used for manufacturing photosensitive materials and magnetic recording media.
2. Description of the Related Art
Photosensitive materials and magnetic recording media are manufactured through a coating step of forming a coating film by applying a coating liquid, such as a magnetic liquid, to a continuously running substrate sheet (hereinafter referred to as “web”). In recent years, magnetic recording media such as magnetic recording tapes particularly for broadcasting and computer use have been rapidly improved in capacity and recording density, and there has been needed a coating technology capable of providing a magnetic layer which has an extremely thin film thickness and a smooth surface.
Coating apparatuses for applying a coating liquid to the surface of a web include, for example, apparatuses of types such as roll coater, gravure coating, roll coating plus doctor roll, extrusion coating, and slide coating. In recent years, an apparatus of extrusion type has been commonly used to apply a magnetic coating liquid.
One method employing an extrusion type coating apparatus in which the tip of a coating head is pushed against a web, as described in Japanese Patent Application Publication No. 58-109162, has been commonly used in the field of manufacturing magnetic recording media because a thin and uniform coating layer can be obtained by increasing the liquid pressure at the tip of the coating head utilizing the web tension, thereby eliminating the air accompanying the web. However, even in the method of applying a single coat of liquid using this type of coating head, the reduction in coating thickness is limited.
As a method for forming a much thinner coating layer, there is a method in which a very thin coating layer can be formed by applying an excess amount of coating liquid to a web using a web-pushing type extrusion coating apparatus and then scraping off the excess coating liquid with a blade disposed on the downstream side of the extrusion coating apparatus, as described in Japanese Patent Application Publication No. 7-287843. In this case, a device which applies an excess amount of coating liquid to a web is not necessarily limited to the web-pushing type extrusion coating apparatus, but coating apparatuses such as roller coater, gravure coater, and extrusion coater equipped with a back-up roll can be used.
In the scraping-off type coating apparatus as disclosed in Japanese Patent Application Publication No. 7-287843, however, since the coating liquid recovered by being scraped off with the blade has once been exposed to the atmosphere in a process from the coating apparatus to the blade, the solvent contained in the coating liquid volatilizes, whereby the liquid physical properties such as viscosity and solid content concentration are changed. Thus, the apparatus has a disadvantage in that the recovered liquid must undergo treatment for adjusting its physical properties such as viscosity and solid content concentration before being reused. Further, the apparatus has a disadvantage in that dust is liable to be included in the coating liquid having been applied to the web because the recovered liquid comes once into contact with the atmosphere, so that the recovered liquid cannot be reused because dust cannot be removed even if an excess liquid recovered with the blade is filtered.
With such a background, the present inventors have proposed coating method and apparatus in Japanese Patent Application Nos. 2001-302401 and 2001-302402 corresponding to U.S. patent application Ser. No. 10/253,427 and German Patent Application No. 102 44 882.5 such that two slits of a coating slit and a recovering slit are provided so that an excess of coating liquid that has been discharged excessively from the coating slit and applied to a web is scraped off with the recovering slit or an excess of coating liquid is sucked up into the recovering slit. According to the coating method and apparatus, the recovered coating liquid can be reused without any one of liquid-adjusting treatment and filtering treatment.
However, the coating apparatus having two slits of the coating slit and recovering slit is a novel coating apparatus exceeding the limit of conventional idea. The optimization of slit clearance of the coating slit and recovering slit is of importance in obtaining a coating layer with a very small and even thickness.